Expression of daily torpor in the Djungarian hamster (Phodopus sungorus) is correlated with the testosterone-driven vasopressin content in the lateral septum.

In the Djungarian hamster (Phodopus sungorus), torpor bouts appear after exposure to short photoperiods (SP) for 11 weeks. Castration accelerates the occurrence of daily torpor when performed between the 1st week before and the 4th week after transfer to SP. In animals castrated 8 weeks before transfer to SP, daily torpor was observed nearly at the same time as in the control group. Vasopressin-immunoreactivity (VP-ir) in the lateral septum (LS) of the Djungarian hamster was examined. Dense VP-ir was seen in sexually active and in sexually inactive hamsters implanted with testosterone (T) capsules. No VP-ir could be detected in sexually inactive animals. Thus, the presence of VP-ir in the LS depends on circulating T levels. In control animals exposed to SP, complete disappearance of VP-ir in the LS was observed after 8 weeks. Castration on the day of transfer to SP reduced this period to 6 weeks. In these animals, the early occurrence of daily torpor is related to the quick disappearance of VP-ir in the LS. When castrated animals were maintained under long photoperiods the disappearance of VP-ir was not complete. However, VP-ir disappeared completely after transfer to SP. The results suggest that the SP message acts on daily torpor indirectly via testicular regression but also 'directly' via VP septal innervation.